The present invention relates to a fluorine gas generating apparatus, and more particularly, to a fluorine gas generating apparatus for producing high purity fluorine gas extremely low in impurity content for the manufacturing process of semiconductors and the like, in particular.
Fluorine gas has been used hitherto as an essential gas in the semiconductor manufacturing field, for example. While the fluorine gas may be used alone, the demand for nitrogen trifluoride gas (hereinafter it is called NF3 gas) synthetically prepared based on the fluorine gas, for the use as a semiconductor cleaning gas or a dry etching gas has been rapidly increased. Neon fluorine gas (hereinafter it is called NeF gas), Argon fluoride gas (hereinafter it is called ArF gas), Krypton fluoride gas (hereinafter it is called KrF gas) and the like are excimer laser oscillation gases used for patterning of a semiconductor integrated circuit. Mixed gas of noble gas and fluorine gas is very often used as raw material of the excimer laser oscillation gas.
The fluorine gas and the NF3 gas used for manufacturing semiconductors and the like are required to be high purity gas containing minute amounts of impurities. In the manufacturing site of semiconductor and the like, it is a common usage that a necessary amount of gas is taken out of a gas container filled with the fluorine gas. Accordingly, it is very important to pay attention to the place to keep the gas cylinder, the assurance of the safety of gas and the preservation of the purity of gas. Further, as the demand for the NF3 gas has been rapidly increased recently, the problem occurs with the supply-side, thus arising the problem that some stock must be backlogged. When considering these problems, it is preferable to place an on-demand or on-site fluorine gas generating apparatus in the location to use it, rather than to treat a high-pressure fluorine gas.
The fluorine gas is usually produced in an electrolytic cell as shown in FIG. 9. Ni, monel, carbon steel and the like are usually used as material of an electrolytic cell body 201. Further, a base plate 212 formed of polytetrafluoroethylene and the like is attached to the bottom of the cell, to prevent hydrogen gas generated and fluorine gas from being mixed with each other. Mixed molten-salt of potassium fluoride-hydrogen fluoride series (hereinafter it is called KFxe2x80x94HF systems) is filled in the electrolytic cell body 201 in the form of an electrolytic bath 202. The electrolytic cell body is separated into an anode chamber 210 and a cathode chamber 211 by a skirt 209 formed of monel and the like. By the application of a voltage between a carbon or nickel (hereinafter it is called Ni) anode 203 contained in the anode chamber 210 and a Ni cathode 204 contained in the cathode chamber 211, the fluorine gas is produced electrolytically. The fluorine gas produced is discharged from a generation port 208 and the hydrogen gas produced at the cathode side is discharged from a hydrogen gas discharge port 207. However, since carbon tetrafluoride gas (hereinafter it is called CF4 gas) produced and hydrogen fluoride gas (hereinafter it is called HF gas) evaporated from the electrolytic bath and the like during the electrolysis come to be mixed in the fluorine gas, it is hard to obtain the fluorine gas of high purity.
Therefore, it is the object to the present invention to provide a fluorine gas generating apparatus that can produce high purity fluorine gas stably.
To solve the problems mentioned above, the present invention provides a fluorine gas generating apparatus for generating fluorine gas of high purity by electrolysis of a mixed molten-salt comprising hydrogen fluoride, the fluorine gas generating apparatus comprising an electrolytic cell which is separated into an anode chamber and a cathode chamber by a partition wall, and pressure keeping means for supplying gas to the anode chamber and the cathode chamber, respectively, to keep an interior of the anode chamber and an interior of the cathode chamber at a certain pressure.
The pressure keeping means permits the anode chamber and the cathode chamber to be always kept at a constant pressure. This permits quick realization of a prescribed concentration and rate of flow of fluorine by introduction of a noble gas of a carrier gas to the fluorine gas. Particularly, this can put the gas in the usable condition quickly from the start up of electrolytic cell. Also, since the interior of the anode chamber and the interior of the cathode chamber are kept at a certain pressure, the prevention of the air and the like from coming into the chambers from outside can be provided, and as such can permit the fluorine gas of high purity to be generated stably. It should be noted that the phrase of xe2x80x9cbeing kept at a certain pressurexe2x80x9d as referred to in the present invention is intended to include the condition of no differential pressure between the internal environment and the external environment (e.g. the use under atmospheric pressure).
The present invention provides a fluorine gas generating apparatus for generating fluorine gas of high purity by electrolysis of a mixed molten-salt comprising hydrogen fluoride, the fluorine gas generating apparatus comprising an electrolytic cell which is separated into an anode chamber and a cathode chamber by a partition wall, pressure keeping means for supplying gas to the anode chamber and the cathode chamber, respectively, to keep an interior of the anode chamber and an interior of the cathode chamber at a certain pressure, a cabinet in which the electrolytic cell is contained and which can provide a controlled atmosphere, and a filter, contained in the cabinet, for filtering out particles in the fluorine gas generated from the electrolytic cell.
This can provide controlled atmosphere around the electrolytic cell, and as such can surely prevent carbon dioxide gas and the like from coming into the electrolytic cell. As a result of this, the generation of CF4 gas produced by reaction of the fluorine gas with the carbon dioxide gas can be suppressed to obtain the fluorine gas of high purity. Also, even if leakage of the fluorine gas from the electrolytic cell occurs, there is no fear of the fluorine gas being leaked outside. In addition, the particles produced by the entrainment from the electrolytic bath during the electrolysis can be surely filtered out by the filter. It is to be noted that the filter preferably has corrosion resistance against the fluorine gas. For example, sintered monel, sintered Hastelloy and the like can be used for the filter. The cabinet for containing the electrolytic cell preferably has corrosion resistance against the fluorine gas. The cabinet is preferably formed, for example, of metal such as carbon steel or polyvinyl chloride.
In the fluorine gas generating apparatus of the present invention, at least one of the anode chamber and the cathode chamber of the electrolytic cell is provided with liquid level detecting means for detecting an upper level and a lower level of liquid level fluctuation of the molten-salt.
This permits the liquid level of the electrolytic bath contained in the electrolytic cell to be grasped even when the interior of the electrolytic cell cannot be visually inspected. This permits the electrolytic bath to be constantly kept at a constant liquid level, and as such can prevent possible back flow of the electrolytic bath. By association of the liquid level detecting means and the power source control means for the electrodes, the electrolysis can be halted whenever the abnormal liquid level of the electrolytic bath is detected.
In the fluorine gas generating apparatus of the present invention, the pressure keeping means is provided with a solenoid valve that is opened and closed based on detection results of the liquid level detecting mean, so as to supply or discharge the gas to and from the interior of the anode chamber and the interior of the cathode chamber.
This permits the automatic supply or discharge of the gas to and from the interior of the anode chamber and/or the interior of the cathode chamber based on the detection results of the liquid level detecting means to detect the liquid level of the electrolytic bath. This enables the liquid level of the electrolytic bath to be always kept at a constant level, and as such can permit the stable generation of the fluorine gas.
In the fluorine gas generating apparatus of the present invention, the mixed molten-salt comprising the hydrogen fluoride is KFxe2x80x94HF systems and there is provided temperature control means for adjusting temperature of the mixed molten-salt comprising the hydrogen fluoride.
This permits the temperature of the mixed molten-salt in the electrolytic cell during the electrolysis to be always kept at a constant temperature, and as such can permit the fluorine gas to be generated efficiently.
In the fluorine gas generating apparatus of the present invention, the gas supplied by the pressure keeping means is a noble gas.
When the generated gas is diluted, for example, with neon gas (Ne gas), argon gas (Ar gas), krypton gas (Kr gas) and the like, that diluted gas can be used as a mixed gas of any selective mixture ratio, and as such can allow the mixed gas to be used as an excimer laser oscillation gas used for patterning of the semiconductor integrated circuits.
In the fluorine gas generating apparatus of the present invention, an anode and a cathode disposed in the anode chamber and the cathode chamber respectively are formed of nickel.
The use of Ni for the anode can prevent drop of the carbon grains caused by the electrolysis using the carbon electrodes. This can prevent the mixture of CF4 by reaction with carbon and fluorine gas, and as such can permit the production of high purity fluorine gas. In addition, this can also prevent the occurrence of the anode effect that is a polarization phenomenon that is typical of the carbon electrode. Further, the use of Ni for the cathode can permit the surface energy to be reduced by hydride and oxide generated on the surface of Ni, as compared with the iron cathode. This permits the bubbles of the hydrogen gas generated to become so large that the mixture with the fluorine gas can be prevented. Also, this can permit the distance between the anode and the cathode to be reduced, and as such can permit the electrolytic cell to be reduced in size.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of metal.
When the metal having high strength and high airtightness, such as Ni, monel, pure iron, and stainless steel, is used for the electrolytic cell body and the coupling, leakage of gas from the electrolytic cell can be prevented. For example, even when the interior of the electrolytic cell is in a helium gas atmosphere under a pressure higher than the atmospheric pressure by 0.1 MPa, leakage of helium gas can be prevented.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is cylindrical in shape.
This permits the electrolytic cell to be heated uniformly from around the circumference by the temperature control means. Also, since the electrodes are disposed concentrically, the current distribution can be made uniform over the electrolytic cell, and as such can permit the stable electrolysis.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of metal and serves as a cathode.
Since the electrolytic cell can serve as the cathode, there is no need to additionally provide the cathode and, as a result of this, the electrolytic cell can be reduced in size. This enables the fluorine gas generating apparatus to be set at any selective location. As a result of this, the fluorine gas generating apparatus is located at any necessary location on a production line in the semiconductor manufacturing process, namely, is set on an on-site basis.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of metal; formed in a cylindrical shape; and serves as the cathode.
This permits the electrolytic cell to be heated uniformly from around the circumference by the temperature control means. Also, since the electrodes are disposed concentrically, the current distribution can be made uniform over the electrolytic cell, and as such can permit the stable electrolysis. Further, since the electrolytic cell can serve as the cathode, there is no need to additionally provide the cathode and, as a result of this, the electrolytic cell can be reduced in size.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of a resin having corrosion resistance against the fluorine gas.
Since the electrolytic cell is formed of the resin having corrosion resistance against the fluorine gas, the electrolytic cell comes to be hard to be corroded by the fluorine gas generated. Particularly, when a little amount of fluorine gas is generated, the electrolytic cell is hardly corroded. It is to be noted here that the structural materials that may be used for the electrolytic cell include fluoropolymer having corrosion resistance against the fluorine gas, such as polytetrafluoroethylene resin, and tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer, trimethylpentene resin and equivalent.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of a resin having corrosion resistance against the fluorine gas and is formed in a rectangular cylindrical shape.
This can provide improved mechanical strength even when the electrolytic cell is formed of resin.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of a resin having corrosion resistance against the fluorine gas and is formed in a rectangular cylindrical shape, and at least one side surface of the electrolytic cell is threadedly engaged with the electrolytic cell so as to be freely opened and closed.
This can permit the facilitation of replacement of the electrodes, and the mixed molten-salt in the electrode and the electrolytic cell and of the electrodes. The threaded engagement of one side surface can provide improved airtightness and improved strength of the electrolytic cell.
In the fluorine gas generating apparatus of the present invention, the electrolytic cell is formed of a resin having corrosion resistance against the fluorine gas and is formed in a rectangular cylindrical shape, and at least one side surface of the electrolytic cell is formed of a transparent resin and the remaining side surfaces are formed of fluoropolymer.
This permits the visual inspection of the interior of the electrolytic cell during the electrolysis, as such can permit the visual inspection of an amount of sludge generated from the electrodes during the electrolysis even when Ni is used for the electrodes in the electrolytic cell. Also, this permits the visual inspection of the liquid level of the electrolytic bath during the electrolysis, as such can permit the control of the liquid level via the liquid level detecting means and the reliable grasping of the information about the liquid level.
In the fluorine gas generating apparatus of the present invention, there is provided a gas line in which the gas passing through the filter is pressured or de-pressured, and there are provided a pressurization apparatus or a depressurization apparatus and storage means in the gas line.
This enables the fluorine gas to be properly regulated to a predetermined pressure and also can prevent the liquid level fluctuation of the electrolytic bath caused by the pressure fluctuation of a reaction system via the pressure regulation valves, and as such can allow a required amount of fluorine gas to be supplied stably.